Body organization of Astacus, Biology tutorial

Introduction:

Astacus can be separated in 3 regions or tagmata, head (consisting of six segments), thorax (eight segments) and abdomen (six segments with posterior telson); head and thorax are about indistinctly fused as cephalothorax covered dorsally by carapace. In Astacus, least specialized appendages, pleopods or swimmerets happen on first five abdominal segments. Exoskeleton of abdomen also illustrates least modification, with each segment comprising of ring formed from dorsal tergite (tergum) and ventral sternite (sternum). Tergites overlap dorsally while sternites are joined by the wide region of flexible membrane. In female Astacus first five abdominal appendages are all like basic structure described except that first pair is reduced in size. In male, first pair is reduced to unjointed rod and second being stiffened, as modifications for sperm transfer. Endopodite of sixth pair of abdominal appendages is altered in both sexes to form broad fan-like structure, uropod whose protopodite has single segment, and both endopodite and exopodite are greatly expanded.

Head has five pairs of appendages. Most anterior are first antennae, or antennules that are relatively short and contain two filaments borne on the peduncle of three segments. Second pair lies posterior to stalked com-pound eye. Endopodite of antennae is represented by the long segmented filament and serves as sense organ; exopodite is basal plate that controls angle at which animal dives. Head is also contains three pairs of feeding appendages surrounding ventrally located mouth. The pair of short heavy mandibles (segment 4) with differing surfaces for grinding covers mouth that has non-flexible labrum at front. Behind these are 2 pairs of accessory feeding structures, first and second maxillae. First maxillae (segment 5) are small and flattened with 2 basal joints which are expanded and fringed with setae, and reduced, unjointed endopodite.

There are 5 pairs of thoracic appendages. Most anterior are first maxillipeds, with the coxopodites and basipodites extended inwards; endopodites form the broad plate and tiny, two-jointed structure, and exopodites are expanded into long, many-jointed filament. Second and third maxillipeds each bear a pair of gills on coxopodite. Exopodite is slender, with several segments, whereas endopodite is large with 5 joints (named from base: ischiopodite, meropodite, carpopodite, propodite, and dactylopodite), each bearing setae. Same joints and gills are found in posterior thoracic appendages, but these all lack exopodites. First pair, chelipeds, are longest and most conspicuous limbs of Astacus, with protopodite and dactylopodite modified as a pair of huge pincers or chelae.

Locomotion:

Astacus usually walks on its walking legs (pereiopods), holding its chelipeds out in front to counter-balance weight of abdomen. Movement is assisted by rhythmic anteriorposterior beat of pleopods (swimmerets) that are mainly utilized for swimming. Unlike several crustaceans, Astacus can't swim using pleopods alone as these are strangely small for its size. If began, it retreats by darting backwards abruptly, consequently of sudden flexure of abdomen with uropods and telson acting together as paddle. Astacus is unable of darting forward.

The digestive system and nutrition:

Astacus contains straight gut with anterior end extremely modified for grinding. There are no cilia, and food is moved completely by peristalsis. Mouth leads in stomodaeum, lined with chitin, and comprising of short oesophagus and voluminous proventriculus, that has two chambers. First (cardiac) chamber has series of chitinous teeth and articulating plates, moved by muscles, that together form gastric mill where food is crushed and ground. Second (pyloric) chamber forms sieve with rows of chitinous setae; here, food is ground in very fine particles before it is passed down alimentary tract in midgut. This grinding protects midgut from damage by abrasion, and also permits Astacus to have very broad and unselective diet.

Astacus comes from hiding to feed at night. Food comprises of almost anything organic, animal or plant, alive or dead; it will eat snails or insect larvae, and often feeds on shells or calcareous algae for the calcareous content. During coldest times of year Astacus retreats in the burrow and doesn't feed. Process of transferring food in mouth engages 6 pairs of appendages (mandibles, maxillae, maxillules, and first, second and third maxillipeds) and thus is complicated. Third maxillipeds are largest and are utilized for choosing and tearing food; apart from its use in food acquisition, it protects other appendages when animal is not feeding. During tearing, torn pieces are grasped by mandibles and pulled by second maxillipeds and with help of first maxillipeds, passed forward to mandibles. Food is then further broken up by forward and backward movements of mandibles, and then pushed in mouth by maxillae.

Ingested food is broken mechanically; chemical digestion takes place in stomodaeum. The large bilobed digestive gland, hepatopancreas that comprises of many finely-branched tubules, lies in haemocoele and opens into midgut. The watery dark-brown mixture of protease, carbohydrase and lipase is generated and sucked forwards in cardiac chamber by dilator muscles, by paired ventral canals protected by setal sieve. Digestion consequently begins in gastric mill, and when it is adequately advanced, partially digested food and fine particles are passed back along ventral canals in pyloric chamber. This is tubular, most anterior region acting for storage, and next as a press in which partly digested and liquid matter is squeezed out. Digested food is absorbed in hepatopancreas that is also utilized for storage of food reserves. Indigestible matter is passed along to midgut and finally leaves anus via cuticle-lined hindgut (proctodaeum). Anus opens in ventral mid-line of telson.

Gaseous exchange:

Astacus has developed the series of outgrowths (epipodites) that function as gills and contain only thin cuticle separating blood from respiratory current, this compensates for limitations of permeable surfaces imposed by the exoskeleton. It contains 18 pairs of gills crowded into the branchial chamber formed by lateral extensions of carapace, called as branchiostegites. Ventral surface of branchial chamber is given by bases of limbs, from second maxilliped to posterior end of thorax. 2 kinds of epipodites happen, that are arranged in 3 layers. Pleurobranchs and arthrobranchs arise from side of the thorax and from articulatory membrane between leg and body, respectively. They both contain feather-like structure with main stem (that carries afferent and efferent blood vessels) bearing closely set branchial filaments on each side. Podobranchs arise from coxopodites of appendages and are double structures; anterior branch is feather-like and posterior plate-like. There is significant variation in arrangement of gills among crustaceans. Water current is necessary for successfully gill function. Current flows forward and is developed by rhythmic sculling of scaphognathites (second maxillae) which has the pumping effect.

Gaseous exchange happens in thin-walled gill filaments that together give the large surface area. Blood has haemocyanin in solution, that functions in a similar way to haemoglobin. Pigment is blue-green in color because of its copper content. Oxygen is taken up and transported, both in combination with haemocyanin and in physical solution. Around 60 to 70% of oxygen available in solution in water current can be taken up by crayfish.

The circulatory system:

Astacus has the open circulatory system, thus its main organs lie and are bathed in blood within blood-filled haemocoel, like in all arthropods. On dorsal side of thorax haemocoel forms the large pericardial space (sinus) surrounding heart. Heart is muscular structure, polygonal in cross section and held in position by 6 strands of elastic fibres. After each contraction of heart muscles these fibres cause it to expand again. Blood enters pericardial sinus laterally from gills, and passes in heart by 3 pairs of ostia (antero-dorsal, lateral, and ventral).

The median anterior artery (ophthalmic artery) extends forwards to supply blood to brain, antennules, and eyes; near its anterior end it has the contractile dilation that functions as the accessory heart. In several small crustaceans this may be only artery present. Two lateral antennary arteries supply antennules, antennae, green glands, and anterior region of alimentary canal. The pair of hepatic arteries leaves heart latero-ventrally, to supply digestive gland and gonads. The median dorsal abdominal artery from posterior end of heart extends along abdomen and supplies abdominal muscles and intestine. Sternal artery also leaves heart posteriorly and descends to ventral region of body where it separates into 2 branches; one of these, ventral thoracic artery, extends anteriorly, and the other, the ventral abdominal artery, extends posteriorly.

Excretory system:

The principal osmoregulatory organs in Astacus are paired green/antennary glands which lie in haemocoele anterior to mouth. They have rudiment of coelom and are at times known as coelomoducts. Glands are composed of the ventral glandular region over which lies large, thin-walled bladder which opens through the duct in base of coxopodite of antenna. Glandular region has inner sac which is partitioned; this leads to green labyrinth of cells from which coiled nephridial canal leads to bladder. Blood is supplied to capillaries within inner sac by antennary artery, and it is though out that ultrafiltration occurs between inner sac and blood; salts are reabsorbed in nephridial canal. Green glands are thus able to regulate salt/water balance in Astacus. Urine formed, that collects in bladder, is hypotonic to blood. Nitrogenous waste, which for most part is ammonia, is expelled through gills. Small amounts of aminonitrogen (10%) and urea are also excreted.

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